Conventionally, a well-known chain guide for a derailleur comprises a pulley plate, pulley shafts fixed to the pulley plate, and pulleys supported rotatably to the pulley shafts respectively. The pulleys each are usually formed in one piece of a metallic material, such as aluminum alloy, or a wear resistant synthetic resin. The pulley plate is supported to a movable member of the derailleur and moves together with the movable member axially with respect to the multi-stage sprockets. A driving chain, which is guided by the pulleys supported to the pulley plate, is switched to one of the multi-stage sprockets. The pulleys, when the chain is switched, move axially of the multi-stage sprockets with respect to the chain through the movable member and pulley plate, whereby teeth of the pulleys contact with link plates of the chain. As a result, a biasing force acts on the pulleys axially thereof, so that a problem is created wherein the pulleys each are subjected to resistance against their rotation making it difficult to rotate smoothly.
The chain, when switched to one of the multi-stage sprockets, changes in its chain line following selection of the sprocket to mesh with the chain, so that the chain, even after switching, interferes at its link plates with teeth of the pulleys. Occasionally, the pulleys are affected by the axial biasing force so as to be subject to resistance against the pulley's smooth rotation.
The pulley's lack of smooth rotation as foregoing causes poor speed control and applies a load against the bicycle's running, so that a cyclist cannot easily drive the bicycle.